Facile synthesis of WS2 nanotubes by sulfurization of tungsten thin films: formation mechanism, and structural and optical properties.

While 2D layers of WS2 have been extensively studied, there are very few investigations of WS2 nanotubes. These have usually been grown via a 2-step process involving a WO3-x intermediate. We report a simple process for the synthesis of WS2 nanotubes via the sulfurization of tungsten films under appropriate conditions and present details of their structural and optical properties that help elucidate the formation mechanism. Electron-beam evaporated films of tungsten are sulfurized under flowing N2 gas at 950-1000 °C temperature under atmospheric pressure to obtain WS2 nanotubes. High-resolution scanning and transmission electron microscopy studies show that 2D WS2 flakes curl up and wrap around themselves to form nanotubes. Interlayer spacings in both 'a' and 'c' directions are slightly smaller than the corresponding values in bulk and thin film WS2. Micro-photoluminescence and micro-transmission studies show a resonance that seems intrinsic to the WS2 nanotubes since it cannot be related to the known optical characteristics of WS2 flakes.

Electronic control of interface ferromagnetic order and exchange-bias in paramagnetic-antiferromagnetic epitaxial bilayers.

The hetero-epitaxially engineered magnetic phases, formed due to entanglement of the spin, charge and lattice degrees of freedom, at the atomically sharp interfaces of complex oxide heterostructures are indispensable for devising multifunctional devices. In the quest for novel and superior spintronics functionalities, we have explored the interface magnetism in the epitaxial bilayer of atypical magnetic and electronic states, i.e., of paramagnetic metallic and antiferromagnetic (AFM) insulating phases. In this framework, we observe an unusually strong ferromagnetic order and large exchange-bias fields generated at the interface of the bilayers of metallic CaRuO3 and AFM insulating manganite. The magnetic moment of the interface ferromagnetic order increases linearly with increasing thickness (7-90 nm) of the metallic CaRuO3 layer. This linear scaling signifying an electronic (non-magnetic) control of the interface magnetism and a non-monotonic dependence of the exchange-bias on metallic layers evolve as novel spintronics attributes in atypical bilayers.

Emergence of nanoscale inhomogeneity in the superconducting state of a homogeneously disordered conventional superconductor.

The notion of spontaneous formation of an inhomogeneous superconducting state is at the heart of most theories attempting to understand the superconducting state in the presence of strong disorder. Using scanning tunneling spectroscopy and high resolution scanning transmission electron microscopy, we experimentally demonstrate that under the competing effects of strong homogeneous disorder and superconducting correlations, the superconducting state of a conventional superconductor, NbN, spontaneously segregates into domains. Tracking these domains as a function of temperature we observe that the superconducting domains persist across the bulk superconducting transition, Tc, and disappear close to the pseudogap temperature, T*, where signatures of superconducting correlations disappear from the tunneling spectrum and the superfluid response of the system.